adjective
Definition: Referring to a single unit or individual, distinct from others.
Example: She is the one person I trust the most.
noun
Definition: A single unit or individual; the first number in counting, represented by the symbol 1.
Example: She gave me a one-dollar bill for my birthday.
pronoun
Definition: A word used to refer to an unspecified individual or thing, often used in a general sense.
Example: One should always try to do their best in any situation.